//页面加载完毕后触发
window.onload=(function(){
    var obtn = document.getElementById('btn');
    //获取可视区域的高度
    var clientHeight= document.documentElement.clientHeight;
    var timer = null;
    var isTop = ture;


   //滚动条滚动时触发
    window.onscroll = function(){
        var osTop=document.documentElement.scrollTop || document.body.scrollTop;
        if (osTop >= clientHeight) {
            obtn.style.display='block';

        }else{
            obtn.style.display:='none';
        };


        if (!isTop){
            clearInterval(timer);
        }
        isTop = false;
    }


    obtn.onclick=(function(){
        //设置定时器
        timer = setInterval(function(){
            //获取滚动条距离顶部的高度
            var osTop=document.documentElement.scrollTop || document.body.scrollTop;
            var ispeed = Math.floor(-osTop / 5);
            document.documentElement.scrollTop=document.body.scrollTop -= osTop+ispeed;
            
            isTop = true;
            console.log('osTop-ispeed');
            if (osTop == 0) {
                clearInterval(timer);
            };
        },30);
    }


}